Electrochemical system

ABSTRACT

An electrochemical system has two separator plates as well as a membrane-electrode assembly arranged at least in regions between the separator plates. The separator plates and the membrane-electrode assembly each have at least two passage openings for a flush arrangement of the separator plates and the membrane-electrode assembly at positioning devices during the assembly of the electrochemical system. At least one resilient bridge is arranged at the periphery of at least one passage opening for a mechanical butting to the at least one positioning device in such a manner that during the stacking of the separator plates and the membrane-electrode assemblies, the membrane-electrode assemblies center themselves in a direction orthogonal to the stack direction between the separator plates.

The present invention relates to an electrochemical system.

The electrochemical system can for instance be a fuel cell system or anelectrochemical compressor system, especially an electrolyzer.Application of an electrical potential to such electrolyzer apart fromthe actual production of hydrogen and oxygen from water, causes thatthese gases are simultaneously compressed under pressure. In addition,electrochemical compressor systems are known which are supplied withwith gaseous molecular hydrogen and in which this gaseous molecularhydrogen is electrochemically compressed when an electrical potential isapplied. This kind of electrochemical compression is especially suitedfor small amounts of hydrogen to be compressed, as a mechanicalcompression of the hydrogen in this case would be much more complex.

A well-known kind of electrochemical systems comprises a stack ofelectrochemical cells with a layering of a plurality of electrochemicalcells, which are separated by separator plates, respectively. Theseparator plates have several tasks:

-   -   Electrical contacting of the electrodes of the individual        electrochemical cells, e.g. the fuel cells, and continuous        conduction of electrical current to the neighboring cell which        allows for a serious circuit of the cells,    -   Supply of the cells with media, e.g. with reaction gases and        removal of the reaction products through a channel structure        which is arranged in an electrochemically active area, the        so-called flowfield,    -   Transfer of the heat produced by the reaction in the        electrochemical cell, as well as    -   Sealing of the different media and cooling channels against one        another and to the outside.

The separator plates may comprise passages for the cooling and/or thesupply and removal of media which serve for the supply and removal ofmedia to the actual electrochemical cells. These electrochemical cellsare for instance membrane electrode assemblies, also referred to as MEA,which MEA each comprise a polymer electrolyte membrane, at least oneelectrochemically active electrode and/or catalyst layer, as well as atleast one gas diffusion layer (GDL), e.g. from metallic or carbonfleece. The GDL points towards the separator plate.

The distribution of gas in these known separator plates is effectedalong the MEA or its GDL using the channel and meander structures on atleast one of the surfaces of the separator plate, for instance on bothsides of the separator plate.

During the production of the electrochemical system, the individuallayers are stacked one on the other. In order to prevent leakages andthus a malfunction of the complete system, it is important that e.g.passage openings are arranged in a flush manner relative to each otherand that no squeezing of individual layers occurs when the individuallayers are compressed tightly. Such squeezing might occur with displacedor wrongly dimensioned layers.

Given the considerable production tolerances of separator plates andMEAs (particularly the outer contour of the MEA), it is necessary in thestate of the art to keep the clearance of these parts large in order toprevent from an undesired overlap of layers, such as an overlap of a GDLon the surface of a MEA with a flank of a sealing structure, e.g. asealing bead in a separator plate. This causes that the geometry of theparts cannot be defined to the degree desired. To reduce the productiontolerances might be an alternative, but it is costly.

In view of this, it is the object of the current invention to providefor an electrochemical system which ascertains a reliable positioning ofthe individual layers with respect to each other, especially a reliablepositioning of the MEAs relative to the separator plates, at low cost.

This object is achieved by an electrochemical system according to claim1 and a configuration according to claim 14, respectively.

The electrochemical system according to the invention, e.g. a fuel cellsystem, comprises at least the following characteristics:

-   -   Two separator plates as well as    -   A membrane-electrode assembly (MEA) arranged between these        separator plates with    -   The MEA being arranged at least in regions between the separator        plates, with    -   The separator plates and the MEA each comprising at least two        passage openings for a flush arrangement of the separator plates        and the MEAs at positioning devices during the assembly of the        electrochemical system and    -   A resilient bridge arranged at the periphery of at least one        passage opening for a mechanical rest on the at least one        positioning device in such a manner that    -   During the stacking of the separator plates and the MEAS, the        MEAs centers themselves in a direction orthogonal to the stack        direction between the separator plates.

The configuration for the production of such an electrochemical systemaccording to the invention comprises at least the followingcharacteristics:

-   -   A positioning apparatus with at least two positioning devices as        well as    -   At least two separator plates and a MEA, with    -   The separator plates and the MEA each comprising at least two        passage openings for a flush arrangement of the separator plates        and the MEAs at positioning devices during the assembly of the        electrochemical system and    -   A resilient bridge arranged at the periphery of at least one        passage opening for a mechanical rest on the at least one        positioning device in such a manner that    -   During the stacking of the separator plates and the MEAS, the        MEAs center themselves in a direction orthogonal to the stack        direction between the separator plates.

With the resilient abutting areas, the MEAs and their adjacent parts canbe designed in such a way that no clearance has to be provided for. Theresilient leaning area or several resilient leaning areas shift the MEA,which is actually a foil-like part, to the centre of tolerance withoutany bending or buckling of the MEA. This way, the MEA remains free ofboth wrinkles and undefined interspaces. If several resilient leaningareas are present, it is preferred that they show identical springrates.

Within a limited range, the MEA also may shift to the correct positionwith respect to the adjacent sealing beads, which causes that the GDLfixed to membrane in a secure way comes to rest in the “bead pocket”. Asa consequence, the size of the GDL can be exactly adapted to the size ofthe “bead pocket”, so that the critical edge areas of the MEAs have abetter support and an increased bypass in the gap between the bead andthe GDL is prevented from.

It is one of the principles of the instant invention that resilientbridges are arranged in a manner that forces between pins and resilientbridges which are directed in the separator plates/the MEAs lead to analignment of a durality of stated separator plates/MEAs.

Advantageous embodiments are described in the dependent claims.

In one embodiment, the polymer electrolyte membrane at least in sectionsat least on one of its surfaces pointing towards a bipolar plate ispermanently connected with a GDL. In this respect, several arrangementsare possible, which are usually symmetric relative to the polymerelectrolyte membrane arranged in the centre. In the state of the art,MEAs with a 5-layer and with a 7-layer design are known. The individuallayers comprise one electrolyte membrane, two electrode layers, two GDLsand—in case of a 7-layer design—two edge-reinforcing foils in thenon-active area of the MEA. In all embodiments, it is characteristicthat the GDL which is laminated on the surface of the MEA does notextend over the complete area of the central layer, thus of the membraneand/or the reinforcing film. Usually, the edge area of the MEA is notcovered with a relatively thick GDL. As a consequence, it is possible toclamp the membrane between the seals of the adjacent separator plateswithout the GDL being compressed by the seals. Usually, the GDL is thenlaterally delimited by the seals, e.g. by sealing beads. In order toachieve a good efficiency of the electrochemical system, it isadvantageous that the distance between the outer edge of the GDL and theseal is small.

The sealing devices already mentioned above are preferably designed insuch a way that they circumferentially delimit an electrochemicallyactive area of the MEA and at the same time encircle the area covered bythe GDL. This way, they delimit the bead pocket already mentioned above.The seal, which is usually circumferential, may be designed in differentmanners. It can for instance be provided as a sealing bead, thus as anintegral structure in the separator plate itself. With a metallicseparator plate, such sealing bead is generally embossed. As analternative, it is also possible to provide the seal as an elastomericbead or rib or as a sealing frame which is inserted into or arranged onthe separator plate.

The separator plate may for instance be realized as a monopolar plate ora bipolar plate. In case of a bipolar plate, the separator platecomprises in fact a pair of joined plates. The separator plate mayconsist in a variety of materials. It can for instance be producedcompletely or in part from metal or plastics. It is also possible thatthe one-layered separator plate or each of the two plates of the bipolarplate is designed as an embossed, one-piece metallic part. For thisembodiment, it is preferred that the channels for the guidance of thefluids of the electrochemical system as well as the seals are embossed,too. It is even possible that all these structures in an individualplate are embossed with a single tool in a single working step. Inaddition, it is possible to provide for a hollow space between the twoplates forming the bipolar plate. This hollow space can for instance beused for the guidance of coolant. The separator plates, dependent ontheir material and the conditions to be met, may have a thickness of0.05 to 0.12 mm, preferably of 0.075 and 0.1 mm, in both cases includingthe limits mentioned. The thickness relates to the material thickness ofthe unformed flat material, preferably of a metal sheet.

The membrane electrode assembly with its 5-layer or 7-layer design mayhave a thickness of 0.1 to 0.7 mm, preferably of 0.15 to 0.4 mm with thelimits being either included or excluded from the ranges mentioned.

A further embodiment provides that the resilient bridge is formed fromthe membrane and/or the reinforcing film and in the stack directionexhibits a thickness of 100 to 500 μm, preferably of 150 to 300 μm withthe limits being either included or excluded from the ranges mentioned.The resilient bridge in its plane orthogonal to the stack directionshows a suiting spring resiliency with a typical width of the bridge of0.5 to 3 mm, preferably of 1.0 to 1.5 mm. The spring resiliency in thisarea is at least as large as to overcome the frictional force of the MEAwhen the MEA lies uncompressed on the active area of the separator plateand to enable a self-centering of the MEA. At the same time the springresiliency is as small as to avoid a deformation of the edge area of theMEA adjacent to the resilient bridge. This way, a warping of thecomplete MEA is avoided.

A further embodiment provides that the resilient bridge is formed at theouter edge of the MEA or that the resilient bridge adjoins to a hole inthe MEA, with this hole preferably not being designed as a passage holefor passing media in the stack direction. It is thus in principlepossible to arrange the resilient bridge(s) on every position of theMEA, both at the periphery of the MEA or in its central areas. It isalso possible but not preferred that the resilient bridge delimits apassage hole for passing media through the MEA. This is possible, but asalready mentioned not preferred given the lack of reproducibility of thecross section of the hole. It is also possible to use a media passagehole as the receiving position for the positioning device, e.g. for aproduction of the plate stack in packages or with particular endplatedesigns.

The number of resilient bridges may vary according to the size of theMEA, its stiffness and the required degree of self-centering. It ispossible to design the MEA with a single resilient bridge. Anotherembodiment provides two resilient bridges arranged on opposite sides andat the periphery of the MEA. It is however also possible to design theMEA with a larger number of resilient bridges, e.g. with six resilientbridges, see FIG. 7 c. These resilient bridges are each positioned insuch a way that they abut to a positioning device, e.g. to a positioningpin. The positioning device itself is part of an apparatus for stackingthe layers.

In the above mentioned configuration for a simple production of anelectrochemical stack according to the invention, the at least oneresilient bridge at the outer edge of the MEA can be provided with suchan oversize, that a permanent pressure butts against a positioningdevice, such as a positioning pin. In this context, the positioningdevice leans to the periphery of the MEA from the outside. Thus, the MEAas such rather shows an oversize which is compensated for by thedisplacement of the resilient bridge. This assures a self-centering ofthe MEA between the at last two positioning devices. An alternativeembodiment provides that at least one resilient bridge is designed insuch a way and can lean to a positioning device arranged in the innerarea of the MEA that the MEA strains itself between two positioningdevices. In this variant, tension causes a self-centering. This is incontrast to the aforementioned example, where the self-centering of theMEA between the positioning devices is achieved by pressure.

In principle, it is possible that not only the MEAS, but the separatorplates are provided with resilient bridges. Given the smaller inherentresiliency of the typical materials of separator plates compared to themembrane materials at the edge of a MEA, the centering effect is muchlarger for a MEA than for a separator plate.

The invention shall now be explained on the example of several figures.It is shown in

FIGS. 1 The general construction of an electrochemical system and 2:according to the invention; FIGS. 3a Partial sectional views ofelectrochemical systems according and 3b: to the invention, whichpartial sectional views show the multi-layer MEA relative to the courseof the lateral beads; FIGS. 4a An illustration of the deficiencies inelectrochemical systems to 4c: according to the state of the art; FIGS.5: A top-view to a MEA according to the invention; FIGS. 6a Anillustration of advantageous geometries in an and 6b: electrochemicalsystem according to the invention; FIGS. 7a Alternative positions ofresilient bridges in MEAs according to 7d to the invention.

FIG. 1 shows an electrochemical system 1. It is designed as a layeringof a plurality of separator plates with a MEA arranged in each of theirinterspaces. This layering is compressed between two endplates, whichcan be identified at the outer edges in FIG. 1, both on the left-handand on the right-hand side. In addition, six fluid conducts are shown.Four of them provide for the supply and efflux of reaction media, theother two realize the supply and release of coolant.

FIG. 2 shows an exploded view of two separator plates, in the exampleshown two bipolar plates 2 and 3 with a MEA 4 arranged between thebipolar plates. The layered construction of the MEA will be furtherconsidered in the context of FIGS. 3 a and 3 b.

Right here in FIG. 2, one can already realize that the electrochemicallyactive area 10 of the electrochemical system which is essentiallycongruent with the area in which the channels of the bipolar plateextend is encircled by a sealing arrangement, e.g. by a bead arrangement9 a. The area in the centre of the bipolar plate which is encircled bythe sealing beads is also referred to as bead pocket.

Details for this are explained in the context of FIGS. 3 a and 3 b.

A section of the bipolar plates 2 and 3 shown in FIG. 2, namely the areaof a port opening 11 is shown in FIG. 3 a. The port opening 11 enablesthe flow of a medium through an electrochemical system. In FIG. 3 a,this flow is indicated with an arrow pointing downwards. Between thebipolar plates 2 and 3, a MEA 4 is arranged. For illustrative purposes,the MEA 4 is depicted as a layering of five layers with a distancebetween the layers. It shall however be mentioned that in the installedstate, these five layers are permanently laminated to each other so thatthey cannot be shifted relative to each other in a horizontal direction,thus in the X-Y plane.

The port opening 11 is sealed towards other areas of the bipolar plateby bead arrangements 9 a. The MEA in its electrochemically active area10 on both surfaces shows GDLs 8. The electrochemically active area 10is thus dimensioned in such a way that the GDLs 8 in the X-Y plane donot touch the bead arrangement 9 a on the right hand side. At the sametime, it has to be ascertained that the GDL 8 approximates the beadarrangement 9 a as closely as possible but does not touch or overlap it.If the GDL 8 protruded too far to the right-hand side, it would besqueezed or bent by the bead arrangement, which has to be prevented. Inorder to achieve a secure positioning of the MEA, the central layer ofthe MEA, the actual membrane, thus a foil, protrudes on the right-handside and shows a passage opening in the area of the port opening 11.This area of the MEA, which means either only the membrane itself of alamination of the reinforcing foils or a lamination of reinforcing foilsand the membrane, is fixed between the bead arrangements 9 a with thepressure resulting from the stacking of the bipolar plates.

FIG. 3 b shows an alternative embodiment. Here, elastomeric seals 9 b,namely elastomeric ribs, are provided instead of the integral beadarrangements 9 a. Apart from this, the arrangement is the same as inFIG. 3 a. The bipolar plates shown in FIGS. 3 a and 3 b are realized aspairs of embossed metallic parts. Other than in FIG. 3 b, the beadarrangements 9 a in FIG. 3 a are integrally embossed, too.

FIGS. 4 a to 4 c are intended for an illustration of the deficiencies ofthe state of the art. These illustrations are more schematic than theones in FIGS. 3 a and 3 b for clarity reasons.

FIG. 4 a shows the situation when producing an electrochemical systemaccording to the state of the art. Here, a layering with a lower MEA 4,a bipolar plate 2 arranged on top of it and an upper MEA 4 is shown.Again, an area with a port opening 11 is shown, which essentiallycorresponds to an edge area of the bipolar plate. The layering isrealized in such a way that one or several positioning devices such asthe positioning pin 6 a shown, are arranged laterally. The actuallayering of the MEAs and the bipolar plates is then performedalternatingly between these positioning devices. These positioningdevices aim on a flush arrangement of all layers of the stack. As isindicated with the dimension arrow heads 13 in FIG. 4 a, it is intendedto keep the clearance between the bead arrangement 9 a and the area ofthe MEA covered with a GDL as small as possible. The dimension arrowheads 13 depict the positional tolerance between MEA and bead. Theexample shown in FIG. 4 a represents a continuous edge of the membrane12 without any weaknesses or bridges.

The problems related to this configuration are illustrated in FIGS. 4 band 4 c.

FIG. 4 b shows a large clearance between the positioning device and theMEA; only the protruding edge of the MEA membrane is shown. This causesthat the bipolar plate and the MEA can shift relative to each other andthat the positional tolerance 13—see FIG. 4 a—is insufficientlymaintained.

A different case is shown in FIG. 4 c. Here a negative clearance isshown, which is indicated with the dimension arrow heads 13. Here, theMEA is too large for the area encircled by the positioning devices 6 aetc., as a consequence, the MEA is forced Sideways, so that it getsundulated and therefore, the positional tolerance 13 as desired—see FIG.4 a—cannot be achieved, neither.

FIG. 5 shows a MEA according to the invention, with which the abovementioned disadvantages can be overcome. The MEA 4, which in the area ofthe bead pocket 9 a of the adjacent separator plate is permanentlylaminated to a GDL 8, on its left and its right edge comprises resilientbridges 7, which are arranged at the outer periphery of the MEA.Openings 14 area provided which are arranged adjacent to the resilientbridges 7 on the side pointing towards the centre of the MEA. Theseopenings 14 enable the resiliency of the bridges. The bridge at itssmallest position has a width of 0.5 to 3.0 mm, preferably 1.0 to 1.5 mmin its plane, thus in the plane of the drawing sheet.

Thus, a resilient bridge is provided at the periphery of at least onepassage hole in the MEA. This resilient bridge allows for a closemechanical contact of the MEA and at least one positioning device, herewith two pins, one on the left-hand side and one on the right hand side.Each pin engages into the notch of a resilient bridge. As a consequence,during the stacking of the separator plates and the MEAs, the MEAcenters itself between the separator plates in a direction perpendicularto the stacking direction. This way, an optimal positioning of the GDL 8in the bead pocket 9 c is achieved. This means that the edge area of theGDL 8 adjoins to the flanks of the bead arrangements 9 a with aclearance as small as possible, see also FIG. 6 a.

FIG. 6 a shows a sectional view according to A-A in FIG. 5. Here, it isobvious that the positional tolerance between the MEA and the bead,reference number 13, is kept minimal. This is mainly caused by themovement of the resilient bridge 7 and its opposite counterpart 7, thusthe bridge 7 on the right-hand side in FIG. 5.

FIG. 6 b represents an enlarged detail, which shows the deflection ofthe bridge, the relative position of the positioning pin 6 a as well asthe tolerance between the MEA, or to be more precise, its outmostmembrane and/or foil section, and the positioning pin 6 a.

FIGS. 7 a to 7 c show variants of the MEA shown in FIG. 5. Here thesurface of the MEA is indicated in a schematic manner only. In thesefigures, only the resilient bridges 7 and their adjacent openings 14 areexplicitly depicted.

FIG. 7 a shows an embodiment with only one resilient bridge per MEA,while the embodiment in FIG. 5 shows two such resilient bridges. Theresilient bridge 7 here interacts with the opposite positioning pin 6 b.

FIG. 7 b represents an embodiment with two resilient bridges as in FIG.5.

For extensive MEAs, an embodiment is preferred which comprises resilientbridges on all its outer edges, as is shown in FIG. 7 c. In the exampleshown here, a total of six resilient bridges are given. Thecorresponding configuration of the production of the electrochemicalsystem may accordingly provide six positioning devices, e.g. six pins,which are positioned accordingly.

FIG. 7 d represents an alternative embodiment with two openings 14 andrespective resilient bridges 7. Unlike, for example, FIG. 7 b, thebridges are configured in a manner that allows positioning pins 6 b tobe arranged inside the openings 14.

1-15. (canceled)
 16. An electrochemical system, comprising: twoseparator plates, as well as a membrane-electrode assembly arranged atleast in regions between the separator plates, with the separator platesand the membrane-electrode assembly each comprising at least two passageopenings for a flush arrangement of the separator plates and themembrane-electrode assembly at positioning devices during the assemblyof the electrochemical system, and at least one resilient bridgearranged at the periphery of at least one passage opening for amechanical butting to the at least one positioning device in such amanner that during the stacking of the separator plates and themembrane-electrode assemblies, the membrane-electrode assemblies centerthemselves in a direction orthogonal to the stack direction between theseparator plates wherein the resilient bridge adjoins to a passage holein the membrane-electrode assembly, where the hole does not serve forthe passage of media.
 17. The electrochemical system according to claim16, wherein the membrane-electrode assembly at least in sections on itsat least one surface pointing towards a separator plate comprises a gasdiffusion layer.
 18. The electrochemical system according to claim 17,wherein at least one of said separator plates comprises a seal,especially a seal for circumferentially delimiting an electrochemicallyactive region of the membrane-electrode assembly.
 19. Theelectrochemical system according to claim 18, wherein the seal encirclesa region of the membrane-electrode assembly which is covered by the gasdiffusion layer.
 20. The electrochemical system according to claim 19,wherein the seal is formed as a sealing bead.
 21. The electrochemicalsystem according to claim 20, wherein the sealing bead is formed as anintegral structure of the separator plate.
 22. The electrochemicalsystem according to claim 21, wherein the sealing bead is embossed inthe separator plate.
 23. The electrochemical system according to claim18, wherein the seal is an elastomeric rib.
 24. The electrochemicalsystem according to claim 18, wherein the seal is an insertion frame.25. The electrochemical system according to claim 17, wherein themembrane-electrode assembly in the area of the gas diffusion layer has athickness of 0.15 to 0.4 mm.
 26. The electrochemical system according toclaim 17, wherein the resilient bridge in a direction orthogonal to thestack direction shows a minimum width of 1 to 1.5 mm.
 27. Theelectrochemical system according to claim 17, wherein the resilientbridge is formed at an outer periphery of the membrane-electrodeassembly.
 28. The electrochemical system according to claim 16, whereinthe membrane-electrode assembly comprises at least two resilient bridgesfor butting to the respective positioning device.
 29. A configurationfor the production of an electrochemical system, comprising: apositioning apparatus with at least two positioning devices as well asat least two separator plates and a membrane-electrode assembly, withthe separator plates and the membrane-electrode assembly each comprisingat least two passage openings for a flush arrangement of the separatorplates and the membrane-electrode assembly at positioning devices duringthe assembly of the electrochemical system and a resilient bridgearranged at the periphery of at least one passage opening for amechanical butting to the at least one positioning device in such amanner that during the stacking of the separator plates and themembrane-electrode assemblies, the membrane-electrode assemblies centerthemselves in a direction orthogonal to the stack direction between theseparator plates.
 30. The configuration according to claim 29, whereinthe outer edge of the membrane-electrode assembly is provided with suchan oversize in the region of the at least one resilient bridge that apermanent pressure is exerted to a positioning device which from theoutside of the membrane-electrode assembly butts to the periphery of themembrane-electrode assembly.
 31. The configuration according to claim29, wherein at least one resilient bridge is designed in such a way thatit is able to butt to a positioning device in an inner area of themembrane-electrode assembly that the membrane-electrode assembly strainsitself between two positioning devices.